Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. Magnetoresistive (MR) sensors are particularly useful as read elements in heads (sensor), used to read magnetically stored data used in the data storage industry for high data recording densities. Three examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), and tunnel junction magnetoresistive (TMR). MR and GMR sensors are deposited as small and thin multi-layered sheet resistors on a structural substrate. The sheet resistors can be coupled to external devices by contact to metal pads which are electrically connected to the sheet resistors. MR sensors provide a high output signal which is not directly related to the head velocity as in the case of inductive read heads.
To achieve the high aerial densities required by the data storage industry, the sensors are made with commensurately small dimensions. The smaller the dimensions, the more sensitive the thin sheet resistors become to damage from spurious current or voltage spike.
A major problem that is encountered during manufacturing, handling and use of MR sheet resistors as magnetic recording transducers is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the sensors, particularly sensors of the thin film type, and the accompanying spurious discharge of the static electricity (ESD) thus generated. Static charges may be externally produced and accumulate on persons or instruments used for performing head manufacturing or testing procedures. These static charges may be discharged through the head causing excessive heating of the sensitive sensors which result in physical or magnetic damage to the sensors.
As described above, when a head is exposed to voltage or current inputs which are larger than that intended under normal operating conditions, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for high recording densities for magnetic tape media (order of 25 Mbytes/cm2) are patterned as resistive sheets of MR and accompanying materials, and will have a combined thickness for the sensor sheets on the order of a few 100s of Angstroms (Å) with a width of 1 to 10 microns (μm) and a height on the order of 1 μm. Future products will use even smaller dimensions. The hard disk storage industry already uses heads with stripe heights and track widths of 0.2 μm or less. Discharge currents of tens of milliamps through such a small resistor can cause severe damage or complete destruction of the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, oxidation of materials at the tape bearing surface (TBS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded. Short time current or voltage pulses which cause extensive physical damage to a sensor are termed electrostatic discharge (ESD) pulses.
Prior solutions to ESD protection can be summarized into two types of approaches: 1) by using diode(s) and 2) by shorting out the sensor element. Both of these approaches have significant disadvantages. Electrically shorting out the MR sensors, by shorting the two ends of the sensor which connect to external devices provides the best possible ESD protection. However, approaches heretofore devised have required either semi-permanent shorting such as removable soldering or specialized and expensive removable components. Furthermore, once the shorts are removed, the MR sensors are susceptible to ESD damage.
In the diode approach, the diode is intended to remain in parallel with the sensor element during normal operation of the system. Simplistically, the diodes function by shunting the ESD current through the diode rather than the MR (sheet resistor) when the voltage across the diodes exceeds a given voltage (Vcrit). Potential problems with the diode approach are: 1) drainage of current under normal operation degrading the sensor performance, 2) excessive weight of the diode package affecting mechanical motion of the tape head, 3) excessive cost of adding a multiplicity of diodes, 4) physically being able to fit a multiplicity of diodes onto a cable, and 5) space constraints within a small tape drive. Another issue is that when the voltage across the diode (Vdiode) exceeds critical voltage, Vcrit, and should be shunting current around the electronic device (ED), the resistance of the diodes can be of the order of 10 ohms. For a device resistance of 100 ohms, the device will shunt 10% of the current. If the current through the diodes is 200 mA, then the current through the device will be 20 mA. For sensitive GMR sensors with thin film layers and small stripe heights, currents of the order of 20 mA can damage the devices. Furthermore, the voltage across the ED will be (approximately) the critical voltage plus the diode current times the device resistance. For the above example, the voltage across the ED will be given by Ohm's law as 2 V. For many sensitive EDs used in the data storage industry, such as TMR sensors, the parts will be damaged with voltages as low as 0.5 to 1 V due to dielectric breakdown.
A need therefore exists for providing protection for electronic devices such as magnetic heads.